Nonwoven fabrics composed of ultra-fine fibers have a large surface area and are excellent in liquid absorbency, softness. (or flexibility), filtration property (or ability) or others, and are widely used in a variety of applications.
Examples of an efficient production process of a nonwoven fabric, being directly related to melt spinning, include a spunbonded process and a meltblown process. A conventional nonwoven fabric composed of continuous (or filament) fibers, produced by a common spunbonded process, has an excellent mechanical strength, but is small in surface area because of large fiber diameter thus short of liquid absorbency, flexibility and filtration property. Compared with the spunbonded nonwoven fabric, a meltblown nonwoven fabric is small in fiber diameter, and as a result is excellent in flexibility and achieves a large surface area. By making use of such properties or abilities, the meltblown nonwoven fabric has been widely utilized for applications such as a wiper material and a filter substrate. However, the meltblown nonwoven fabric is low in mechanical strength by itself, and therefore is generally used by laminating a spunbonded nonwoven fabric or the like as a supporting layer thereon.
Moreover, a process is known as a production process of a nonwoven fabric composed of ultra-fine continuous fibers, where the process comprises subjecting a nonwoven fabric composed of conjugate continuous fibers of two or more kinds of polymers to separate or split application along the direction of fiber length by a physical or chemical technique to transform thus obtained conjugate continuous fibers into ultra-fine continuous fibers. However, in this process, two or more kinds of polymers are present in the nonwoven fabric. Thus, a nonwoven fabric composed of ultra-fine continuous fibers of only one polymer, can be obtained by removing the other polymer(s) with the use of chemical(s). However, since the remaining polymer without being removed is adversely affected in the removing process, a combination of polymers constituting the conjugate fiber is limited to a specific one in many cases.
On the other hand, a polyvinyl alcohol (hereinafter the term is sometimes abbreviated PVA) is a water-soluble polymer, and it is known that the degree of water solubility in the PVA can be changed based on a basic bone structure thereof, a molecular structure thereof, a form thereof and various modification. Further, the PVA is identified as having biodegradability. The harmony between synthetic products and natural world has been a major problem recently in global environment, and the PVA and PVA-series fibers having such basic performances have become a center of attraction.
The inventors of the present invention proposed in Japanese Patent Application Laid-Open No. 262456/2001 (JP-2001-262456A) a process for producing conjugate continuous fibers composed of a PVA and other thermoplastic polymer by melt spinning and simultaneously making the obtained conjugate continuous fibers into a nonwoven fabric; and a nonwoven fabric composed of continuous fibers, having a modified cross-sectional form (or shape) or a ultra-fine fineness, obtained by extractive removing the PVA from the nonwoven fabric with water.
However, this document is silent on retaining part of the PVA in the fabric. Moreover, the document also silent that a nonwoven fabric composed of the conjugate continuous fibers, having a water absorbency with a high durability unpredictable from ordinary common sense, can be obtained depending on the condition for retaining the PVA. The conventional extractive treatment condition with water, that is, a method which comprises repeating an extractive treatment using a hot water and a severe stirring many times, and further dry treating at heat temperatures, cannot provide the water absorbency with a high durability.